[Bug] Incomplete documentation for using BigQuery DataFrames (python models)

### Is this a new bug?

- [x] I believe this is a new bug
- [x] I have searched the existing issues, and I could not find an existing issue for this bug

### Which packages are affected?

- [ ] dbt-adapters
- [ ] dbt-tests-adapter
- [ ] dbt-athena
- [ ] dbt-athena-community
- [x] dbt-bigquery
- [ ] dbt-postgres
- [ ] dbt-redshift
- [ ] dbt-snowflake
- [ ] dbt-spark

### Current Behavior

Here are the issues I've found while trying to follow the docs:
- missing info that setting `submission_method` to `bigframes` in `dbt_project.yml` or model config is required to use this feature instead of Dataproc, as described in https://cloud.google.com/bigquery/docs/dataframes-dbt#configurations (without setting this, an error is shown that you need to enable Dataproc API)
- it's unclear why `compute_region` should be specified when using oauth (shouldn't it be read automatically, same as the project?), and also, which region to specify (BigQuery has no region, and GCS buckets can be multi-region, eg. region can be `us`, while no such region exists as an option for `compute_region`)
- looks like enabling Vertex AI API is required:
```python
403 Vertex AI API has not been used in project my-project before or it is disabled. Enable it by visiting https://console.developers.google.com/apis/api/aiplatform.googleapis.com/overview?project=my-project then retry. If you enabled this API recently, wait a few minutes for the action to propagate to our systems and retry.
```
It would also be great to know how/why dbt uses the Vertex AI service, because it's hard to understand what the connection may be if you're not already familiar with Vertex AI.

### Expected Behavior

Would be nice to be able to use BigQuery Dataframes instead of Dataproc for python models.

### Steps To Reproduce

Try to follow the instructions in dbt docs: https://docs.getdbt.com/docs/core/connect-data-platform/bigquery-setup

### Relevant log output

```shell

```

### Environment

```markdown
- OS:
- Python:
- dbt-adapters:
- :
```

### Additional Context

_No response_